The present invention generally relates to remote telephone switching and testing and, more particularly, to DTMF (dual-tone multi-frequency) activated apparatus which provides a customer who leases a private telephone line from a telephone company, or the telephone company itself, with a highly flexible and expandable circuit test capability.
In our prior U.S. Pat. No. 4,258,236, we disclosed a remote telephone line switching and testing circuit for a four-wire communication system providing communication between equipment at a first location and equipment of at least a second remote location. The switching and testing circuit is adapted to be connected to the receive pair of a four-wire telephone line at the remote location and includes a tone detecting circuit which is responsive to tones of a pre-determined frequency for providing output detection signals. A control circuit is responsive to those output detection signals to generate control signals.
According to one aspect of the invention disclosed in our prior patent, switching circuitry is provided which is responsive to a first control signal from the control circuit for disconnecting the equipment at the second location and providing quiet terminations for both the receive and transmit pairs of the four-wire line. This is the data-streaming dropout mode. The switching circuitry is further responsive to a second control signal from the control circuit and for re-establishing the connection between the equipment at the second location and the four-wire line.
According to another aspect of the invention disclosed in our prior patent, switching circuitry is provided which is responsive to a first control signal from the control circuit for disconnecting the equipment at the second location from both pairs of the four-wire line and connecting a milliwatt test signal source to the transmit pair. The switching circuitry is further responsive to a second control signal from the control circuit for disconnecting the milliwatt test signal source from the transmit pair and reconnecting the equipment at the second location to the four-wire line.
In the preferred embodiments of the invention disclosed in our prior patent, the control circuit is a sequencer circuit including a two-stage binary counter and decoding logic. In the first preferred embodiment, the counter counts three detection signals and is then automatically reset after a brief delay. On the first count output, the switching circuitry disconnects the equipment at the second location from the four-wire line and connects a loop-back amplifier between the receive pair and the transmit pair. On the second count output, the switching circuitry disconnects the loop-back amplifier and provides quiet termination for each of the receive and transmit pairs. On the third count output, the loop-back amplifier is briefly reconnected between the receive pair and the transmit pair to provide a brief tone pulse, and thereafter the loop-back amplifier is disconnected and the equipment at the second location is reconnected to the four-wire line. At this time, the counter is reset to be ready for the next cycle of switching and testing.
In the second preferred embodiment of the invention disclosed in our prior patent, the counter counts four detection signals, and since this count results in both stages of the counter being in their initial states, there is no need to provide a reset for the counter. On the first count output, the switching circuitry disconnects the equipment at the second location from the four-wire line and connects a loop-back amplifier between the receive pair and the transmit pair. On the second count output, the switching circuitry disconnects the loop-back amplifier and provides quiet terminations for each of the receive and transmit pairs. On the third count output, a milliwatt test signal source is connected to the transmit pair. On the fourth count output, the milliwatt test signal source is disconnected and the connection of the equipment at the second location to the four-wire line is re-established.
In the case where the second location includes a plurality of modems, the invention of our prior patent provides a separate switching and testing circuit for each modem. Each switching and testing circuit is assigned a different pre-determined tone frequency to which only its tone detecting circuit is responsive. With this arrangement, it is possible to separately address each switching and testing circuit to provide a multi-point testing capability. The tone generator to which the tone detecting circuit is responsive can be at any convenient location referred to simply as the first location, and if multi-point capability is provided, the tone generator is a multi-tone generator with any one of a pre-determined number of tone frequencies being selectable. In actual practice, the multi-tone generator produces frequencies between 300 Hz and 3 KHz in intervals of 100 Hz resulting in a total of thirty tones, and hence modems, which may be selected. This is usually entirely adequate in the case where a single remote customer premise includes a plurality of data collecting and/or processing equipment which are connected by means of modems to the telephone line, and often adequate in the case where switching and testing circuits are to be located at a plurality of remote customer premises. However, in this latter case, there are a number of applications, such as banking applications, for example, where the number of branch banks connected to a central bank by telephone line may be several hundred. In such applications, the multi-tone generator is not adequate to address the number of remote switching and testing circuits.